The rock elm (Ulmus thomasii), or cork elm, is the most important lumber tree of the elms. It is smaller than the American elm and has a straight trunk and oblong crown.
Habitat: Grows at the bases of moist wooded slopes. Found mainly in southeastern Iowa.
Hardiness: Zone 3
Growth Rate: Moderate
Mature Shape: Cylindrical crown with upright short branches. Is narrower than most elms.
Height: 80-100 feet
Width: 70-90 feet
Site Requirements: Regularly found on moist well-drained sandy loam, loam, or silt loam soils in mixture with other hardwoods.
Leaves: Alternate, simple, double-toothed with unequal leaf bases
Flowering Dates: March - May
Seed Dispersal Dates: May - June
Seed Bearing Age: 20 years
Seed Bearing Frequency: Every 3 to 4 years
Seed Stratification: No stratification period is needed.
The leaves are 2-1/2 to 4 inches long, smooth above, hairy below. Leaves are shiny dark green above and light green below. The larger twigs have corky, irregular, prominent ridges. The brown buds are broadly rounded and velvety. The characteristic corky ridges are present on the small trunks and branches and on larger trunks the bark is broken into narrow, interlacing ridges. The wafer-like fruit is winged, contains one seed and is similar to the fruit of other elms but is narrower and velvety.